1. The Field of the Invention
The present invention is directed generally to a Vertical/Short Take-Off and Landing (V/STOL) aircraft, and more particularly, to a twin-fuselage propeller equipped V/STOL aircraft capable of performing take-off and landing with its thrust vector vertically or horizontally disposed.
2. Background Art
V/STOL aircraft generally rely on rotating the thrust vector between horizontal and vertical directions to take advantage of limited space for take-off and landing while still realizing the economy of conventional forward flight for some segments of a flight. So-called tilt-rotor/tilt-propeller aircraft are capable of operating through a range of angles between horizontal and vertical where the direction of travel is determined by the angle of the thrust generated by the tilt-rotor or tilt-propeller. When flying in a conventional manner, lift is provided by wings. For hover or vertical translation, rotors are used to supply lift.
Stability is a pernicious problem in V/STOL aircraft. A few configurations have been tested such as Canadair CL-84 and Ling-Temco-Vought XC-142 (hereinafter CL-84 and XC-142). Rotors, which swivel, have been mounted on wing tips, such as in Bell Boeing V-22 (hereinafter V-22). These designs suffer from instability caused by thrust imbalances along the vertical axis, i.e., the aircraft's equilibrium is unstable. Attempts to alleviate instability have led to cumbersome, complex and expensive aircraft. Tilt-rotor aircraft are essentially a hybrid between conventional aircraft and helicopters. They are capable of rapid forward flight, as well as slower vertical translation. However, as they are sustained by lift from wings in forward flight, tilt-rotor aircraft fly much faster and more economically than helicopters.
German Pat. No. DE4,443,731 (hereinafter Conrado) discloses a V/STOL aircraft incorporating an engine having a drive unit that is an airscrew drive. The airscrew drive has two sets of coaxial counter-running cyclically and collectively adjustable rotary blades. Conrado differs from the present invention in two major respects. First, Conrado discloses an engine that is coupled to an inboard winglet and together they are configured to be tiltable as a unit. In certain flight segments, the winglet is configured to tilt until the propeller axis is vertical. For stability, the winglet must be designed to withstand the rigors of vertical-horizontal or horizontal-vertical flight transitions, and therefore requiring substantially large structure to support such winglet. Therefore downwash from rotor blades can be substantially blocked by the winglet. Secondly, Conrado's engines are placed between the two fuselages, resulting in the requirement of an unnecessarily large gap between the fuselages or a pair of propellers having unnecessarily small diameter due to the spatial limitation of the gap. It shall be apparent to those skilled in the art, the diameter of the propellers, especially those of shallow pitch (designed for vertical flight), must be configured sufficiently large for efficient propulsion. However, the gap is not to be configured so large as to significantly lower structural rigidity. Whereas for forward propulsion, the diameter of the propellers shall be smaller but with sharp pitch. These are contrary requirements. In Conrado, the smaller propeller diameter necessitated by the gap between the fuselages forces the use of a small propeller diameter, sacrificing lift efficiency, which must be compensated by an increase in power.
U.S. Pat. No. 5,758,844 to Cummings (hereinafter Cummings) discloses a vehicle including a fuselage, a plurality of lifting surfaces attached to the fuselage having control devices attached thereto and an articulated propulsion system attached to the fuselage. The propulsion system includes a duct assembly pivotably connected to the fuselage. The duct assembly includes a duct and a propeller assembly mounted within the duct. A motor assembly is connected to the propeller assembly. The duct assembly may be positioned in a substantially vertical position to provide sufficient direct vertical thrust for vertical take-off and landing and may be directed in other positions to provide a varying spectrum of take-off and landing configurations, as well as a substantially horizontal position for high speed horizontal flight. During horizontal flight, the wings provide the lift, which is more efficient than a propeller providing lift. Cummings takes advantage of a center line propulsion, so that there are no asymmetric propulsion loads, similar to the concept of the present invention. Cummings differs in at least two significant ways however. The propulsion system of the present aircraft includes a twin counter-rotating propellers without a shroud while Cummings' propulsion system is a ducted fan. As it shall be appreciated by those skilled in the art, ducted fans are known to be inferior to propellers in the following ways: (a) Ducted fans are less efficient than a propeller at cruise (at lower thrust level); (b) Good efficiency requires very small clearances between the blade tips and the duct, where such clearances are difficult to maintain; (c) Ducted fans require high RPM and minimal vibration; (d) Complex duct design, and weight increase even if constructed from advanced composites; (e) At an angle of incidence greater than about 30 degrees, parts of the duct would be stalled and producing drag; and (f) Articulated wings complicate the control of vertical-forward or forward-vertical flight transitions. As ducted fans are generally smaller in diameter than propellers to create equivalent amounts of lift, the use of a pyramid structure to elevate the propulsion system is not contemplated in Cummings. Further, no articulated wings or winglets are utilized in the present invention, thereby reducing the complexity of the present aircraft and allowing the use of more than one wingtip-to-wingtip spar for added structural rigidity.
U.S. Pat. No. 3,159,361 to Weiland discloses an aircraft having four articulated engines distributed on four locations of the aircraft. In Weiland, there is neither consideration in making the propellers as large as possible with respect to the fuselages, nor consideration of consolidating the multiple thrust vectors into a single thrust vector.
Russian Pat. Nos. RU2078715C1 and RU2075425C1 to Arabi Mukhamed Yu (hereinafter Yu) disclose two versions of twin-fuselage passenger aircraft. However, Yu does not contemplate vertical flight.